Motion control engineers have long struggled with determining the correct wiring for phase and hall sensor relationships between three phase brushless DC motors and amplifiers. The problem becomes significant for development efforts on complex machines that require a plurality of different motors, amplifiers and manufacturers to adequately satisfy motion requirements. To aggravate the problem further, there is no standard nomenclature between hall and phase connections between motor and amplifier vendors. For three hall sensor wires, there are six possible connections between the motor and amplifier. Similarly, for three phase wires, there are six possible connections between the motor and amplifier. The net result is that there are 36 possible unique wiring combinations of which only six are correct. However, once any combination has been chosen for the hall wires, the problem becomes determining which one of the six possible motor combinations is correct (it is equally valid to connect the motor first and then determine which one of the six hall combinations is correct).
For correct motor phasing, current must be applied to each motor phase by the amplifier at the same moment in time that the back-electro-motive-force or BEMF, measured as voltage, for that motor phase is at a peak. A mechanical analogy is firing a spark plug when the piston is at the top of its stroke.
Conventional phasing methods use a trial-and-error approach in which the halls are attached to the amplifier, and then the correct motor wiring is determined by finding the combination that seems to run the best. Of the six possible combinations for a single set of hall connections, three of these will result in rotation that is the opposite of the hall signal rotation pattern and will not work at all. Of the remaining three, one will not turn the motor at all (current will flow through the windings but it will produce no torque), one will run the motor at reduced torque and one will be the correct connection. Trial-and-error methods have been demonstrated to be subject to error because it is sometimes difficult to determine which combination is best without the use of a dynomometer. In many cases, two out of the six possible phase wiring combinations will appear to run the motor satisfactorily, but only one is correct.
Phasing problems are particularly apparent in machines, such as high speed inserting machines for mass producing mailings, which use many brushless DC servo motors. In such machines, technicians may incorrectly phase one or more motor applications. Such incorrectly phased applications can commutate improperly for several months, resulting in elevated motor temperature and occasional software initiated motor stoppages due to excessive position error. Such stoppages can be incorrectly attributed to intermittent motor encoder failures because the motor""s encoder value might intermittently fail to change when the motor is commanded to perform an aggressive acceleration. However, the encoder is not to blame when the rotor has become stalled at an angular position where a commutation switch point occurred. Incorrect commutation can result in reduced generated torque at a particular rotor position and the reduced torque might not overcome the sum of the motor cogging torque and friction load torque, resulting in rotor stall.
Using the present invention, proper phase wiring between the motor and amplifier can be determined without using trial-and-error techniques. The present invention requires that the user know the BEMF waveforms and hall sensor output relationships for the motor, and the phase current output waveforms and hall sensor input relationships for the amplifier. These relationships are typically depicted as a function of the rotor positions in electrical degrees. Proper phase wiring between the motor and the amplifier can be achieved by reconciling desired rotor positions that are commonly described by the known characteristics of both the motor and amplifier.
In accordance with the present invention, proper phase wiring is achieved using the following steps. First, hook the three hall sensor signal wires from the motor to the amplifier in any order. Next, referring to the known characteristics of the motor, for a first selected hall sensor find out which two motor phases (and their corresponding connections) produce a BEMF peak at the same rotor position (in electrical degrees) as the middle of a peak of the waveform for the first hall sensor signal. The polarity is not important at all, as a negative peak is just as good as a positive.
Then, referring to the known characteristics of the amplifier, for a first hall sensor input connected to the first hall sensor in the motor, determine which two amplifier phase connection pins are intended to provide current during the middle of a peak in the waveform for the first hall sensor input signal. Again, the polarity does not matter.
Based on these observations, it is determined that the two identified motor phase connections should be connected to the two identified amplifier pins, but it is not known which is which. However, regardless of the polarity, it is known that the third motor phase connection needs to be connected to the third amplifier pin because they form the unused phase connection for the portion of the motor""s electrical cycle under consideration. Accordingly, the third motor phase connection should be connected to the third amplifier pin.
Next, this process is repeated for a second selected hall sensor. Again, for another portion of the motor""s electrical cycle at the middle of a peak in the waveform for the second hall sensor""s signal, two sets of motor and amplifier connections will be identified as providing current. The unused phase connection for that portion of the motor""s electrical cycle will also be identified. Accordingly, the unused motor phase connection should be connected to the unused amplifier pin.
By process of elimination, the final unconnected motor phase connection must connect to the final amplifier phase connection pin. However, if desired the above process can be repeated with respect to the third and final hall sensor.